EP0000886B1 - Process and device for producing a reaction mixture resulting in foams or homogeneous substances and for its subsequent introduction into a cavity - Google Patents

Description

The invention relates to a method and a device for producing a foam or homogeneous reaction mixture from at least two liquid reaction components and for its subsequent introduction into a cavity, the reaction components being introduced into a mixing zone and mixed with one another, and the mixture then being passed through a even during the outflow process, the automatically adjustable outflow cross section is passed through and filled into a cavity. To carry out the method, a device is assumed which consists of a mixing chamber arranged in a housing and which has inlet openings for the reaction components and an outlet opening for the reaction mixture, this outlet opening being arranged opposite a resiliently mounted baffle plate which is adjustable in the axial direction of the outlet opening.

The production of molded parts from foam, structural foam, integral foam, in particular based on polyurethane, but also from homogeneous materials, has reached a high technical level with regard to the possible applications. In particular, more and more molded parts are being made from the materials mentioned, which have visible surfaces or which must have high strength properties. The discharge of the liquid reaction mixture from the mixing chamber and the subsequent entry into the mold have a particularly critical effect on the nature of the molded parts. In particular, higher-viscosity reaction mixtures cause difficulties here. The problem is exacerbated by the fact that solid additives in powder form or small-grained form have to be introduced with the reaction mixture into the cavity of the molding tool to an ever greater extent. These additives or fillers generally increase the viscosity. One tries in various ways to avoid that parts of the reaction components get unmixed into the cavity; that air is hammered into the reaction mixture on the way from the mixing chamber to the cavity and in the cavity itself; that there are delays that could cause streaks; and that an uneven flow front of the reaction mixture builds up. It is generally known that this problem can be overcome by a flow of the reaction mixture that is as laminar as possible.

Various solutions have been found, which are sufficient in individual cases. The simplest way to help is by allowing the reaction mixture to flow into the open mold so slowly that it flows laminarly over a plane that is as slightly inclined as possible to the lowest point of the mold. Passing on and cascading flow must be avoided. A rational production method is not possible with this. This procedure is also not suitable for entry into closed molds because the filling process cannot be observed. Other solutions use a specially shaped sprue. As a result, one is bound to certain operating conditions, such as the flow rate and viscosity of the reaction mixture, so that the device cannot be used universally. Therefore, it has also been proposed to arrange the sprue in an interchangeable part, so that the same device can be operated with different sprue under different conditions. This also means an increased effort.

Finally, a device has become known in which an adjustable baffle plate is arranged opposite the outlet opening of the mixing chamber. The distance of the baffle plate from the outflow opening is adjustable, so that a perfect source flow is achieved in this way. The distance is fixed during the filling process. However, it is possible to retract the baffle plate immediately before the filling process is completed, so that it forms a plane with the wall of the molding tool in which it is arranged. As a result, the entire space between the outlet opening and the baffle plate is completely filled when the filling process is complete; because in this known device the space between the outlet opening of the mixing chamber and the baffle plate also belongs to the cavity of the mold, so that the molded part also has a suitable wall thickness in this section (DE-A-2 348 658 and US-A-3 991 147).

The object of the invention is to find a method and a device which ensure the production of perfect molded parts even under changing operating conditions, as are often required when the throughput quantity or the viscosity of the reaction mixture changes.

In terms of process engineering, this object is achieved in accordance with the invention in that the mixing zone is closed before the reaction components are introduced and is only opened automatically at a predetermined pressure value by the pressure built up therein by the introduction of the reaction components.

This ensures that, especially at the beginning of the mixing process, the reaction mixture can only flow out of the mixing chamber when it is completely full. Should open one of the reaction components when opening the inlet organs ten ahead, there is no danger that this portion is entered unmixed into the cavity to be filled, since such turbulence in the sealed mixing chamber finely distributes such a portion and thus makes it harmless. Especially at the beginning of the outflow process, the automatic adjustability of the outflow cross-section is particularly noticeable because the outflow speed remains constant in accordance with the set pressure. The same applies to the end of the outflow process. The method according to the invention thus makes it possible to keep the flow rate of the reaction mixture constant in all critical phases, this constancy being a prerequisite for maintaining the laminar flow.

According to a special embodiment of the method according to the invention, the size of the outflow cross section is fixed towards the end of the outflow process.

This ensures that - provided that the space located in the area of the outflow cross section is also to be filled - it is free of the closure element of the mixing zone.

The device for carrying out the method is characterized according to the invention in that the baffle plate closes the outlet opening before the beginning of the mixing process.

It is thereby achieved that the outlet opening of the mixing chamber is sealed before the beginning of the mixing process. Particularly in the case of the generally known mixing heads with ejection pistons, in which towards the end of the mixing process the ejection piston completely pushes the mixture out of the mixing chamber and thus cleans it, the invention has proven to be advantageous because any air is also displaced from the mixing chamber by using the baffle plate, the outlet opening is sealed as long as the ejection piston is still in the ejection position. To return the ejection piston, an increased power requirement is then necessary because a vacuum is created in the mixing chamber that is now being formed; on the other hand, however, the reaction mixture is no longer enriched with the air that is otherwise in the mixing chamber. The resilience of the baffle plate is adjustable so that it automatically opens at a predetermined pressure prevailing in the mixing chamber and the outflow cross section is also set accordingly automatically.

From DE-A-24 13 337 it is known to use a piston which is displaceably mounted in relation to the mixing chamber, is under preload and can be pushed into the mixing chamber, in order to keep the mixing chamber volume small at the beginning of the mixing process. When the mixing chamber pressure rises, the piston automatically returns from the mixing chamber, insofar as this allows the set preload. At the end of the mixing process, the lock that prevents the piston from moving further backward is released, so that the mixing chamber is completely released from the piston.

With this device, the mixing chamber is not sealed off from the outside at the beginning of the mixing process, and if one of the components advances, the unmixed part gets into the mold cavity and causes defects in the finished molded part. This device has therefore been improved in that a hollow piston is used, in the cavity of which the first portions of the components are collected when they enter the mixing chamber and mix thoroughly with one another and with the rest of the mixture in the course of the mixing process. However, this embodiment is very complex and very problematic because of the required sealing surfaces of the hollow piston.

DE-A-25 44 749 shows a mixing device with a sprue, a displaceable bluff body being provided in this sprue, the position of which can be adjusted in relation to the outlet opening of the mixing chamber such that a defined back pressure is maintained in the mixing chamber. In this embodiment, however, it is not possible to close the outlet opening of the mixing chamber before the mixing process begins.

Finally, DE-A-19 48 455 is also known, which relates to a mixing device combined with a molding tool. There, the inlet opening of the mold cavity can be closed by an elastic flap that opens under the pressure of the material flowing in from the mixing chamber. This flap does not have any effect on the mixing chamber, since the sprue channel lies between the mixing chamber and the mold cavity.

The baffle plate is preferably mounted on a spring. The easiest way is to use a mechanical spring, but hydraulic or pneumatic springs are also suitable. The spring is held under prestress, this prestress corresponding to the pressure which has to build up in the mixing chamber so that the outlet opening is released. By using springs with a wide variety of characteristics or simultaneous use of several nested springs of different spring constants, the spring travel which can be influenced in this way can achieve a wide variety of effects which, in the individual case, with appropriate selection, permit the laminar entry of the reaction mixture into the cavity under the most varied of conditions.

According to a special embodiment of the device according to the invention, the spring rests on an adjusting screw guided in the housing. This makes it particularly easy to set and change the preload.

After another special execution The baffle plate is assigned a fixing device. This consists, for example, of a hole arranged laterally in the baffle plate, into which a pin which is mounted in the housing and is under spring tension can engage. This embodiment is particularly advantageous when the cavity is that of a molding tool and the area between the outlet opening and the baffle plate also belongs to the area of this cavity in which a molding is formed. As a rule, the known mixing head with ejection piston is used here, which pushes the mixture residue from the mixing chamber into the mold cavity towards the end of the filling process. At the same time during this process, the baffle plate is pushed back so far that it forms a plane with the associated mold wall, the fixing device coming into operation in this position. Their position must of course be determined in relation to the desired position of the baffle plate.

This embodiment is particularly cheap, because with a correspondingly favorable arrangement of the mold cavity to the outlet opening, the use of the baffle plate which is automatically adjustable according to the invention not only enables the production of cast-free, but even marking-free molded parts, because the marking coincides with an edge of the molded part.

The device according to the invention can be designed for a wide variety of fields of application. The embodiment as an ejection piston mixing head which is arranged on a molding tool has already been described above. The baffle plate according to the invention can also be attached to the so-called hand mixer head, which is primarily used for insertion into open molding tools. There is no need for post-mixers. The baffle plate also replaces the otherwise usual means, such as perforated disks or deflection cones, with which the required dynamic pressure is built up in the mixing chamber. Bie devices that are suitable for the production of extruded profiles, the baffle plate according to the invention has the advantage that the length of the mold required for the formation of the extruded profile is reduced because the flow speeds are reduced in the initial area.

• Fig. 1 die Vorrichtung des ersten Ausführungsbeispiels im Längsschnitt gemäß Linie A-B in Fig. 2,
• Fig. 2 die Vorrichtung des ersten Ausführungsbeispiels im Querschnitt gemäß Linie C-D-nach Fig. 1,
• Fig. 3 die Vorrichtung des ersten Ausführungsbeispiels im Längsschnitt gemäß Linie E-F in Fig. 2,
• Fig. 4 die Vorrichtung des zweiten Ausführungsbeispiels im Längsschnitt,
• Fig. 5 das Formkastenteil der Vorrichtung gemäß dem zweiten Ausführungsbeispiel, an dem der Mischkopf angeordnet ist, in der Draufsicht auf die Trennebene und,
• Fig. 6 das Formkastenteil der Vorrichtung gemäß dem zweiten Ausführungsbeispiel, an dem die Prallplatte angeordnet ist, in der Draufsicht auf die Trennebene.

In one drawing, the device according to the invention is shown purely schematically using two examples of a mixing head which works with an ejection piston and is combined with a molding tool and is explained in more detail below. Show it:

• 1 shows the device of the first embodiment in longitudinal section along line AB in Fig. 2,
• 2 shows the device of the first embodiment in cross section along line CD-in FIG. 1,
• 3 shows the device of the first embodiment in longitudinal section along line EF in FIG. 2,
• 4 shows the device of the second embodiment in longitudinal section,
• 5 the molding box part of the device according to the second exemplary embodiment, on which the mixing head is arranged, in a top view of the parting plane, and
• Fig. 6, the molding box part of the device according to the second embodiment, on which the baffle plate is arranged, in a plan view of the parting plane.

1 to 3:

A mixing head 1 is combined with a molding tool 2. A housing 4 for a bore 5 is attached to the upper mold part 3. Inlet openings 6, 7 point into this bore 5. In the bore 5, an ejection piston 8 is guided, which in the mixed position releases the inlet openings 6, 7, so that the mixing chamber is located between the inlet openings 6, 7 and the outlet opening 9 located at the end of the bore 5 10 is formed. A mold part plane 11, which runs at the level of the outlet opening 9, separates the upper part 3 of the mold and the lower part 12 of the mold. A baffle plate 13 is arranged opposite the outlet opening 9 in the lower mold part 12. It is guided in a bore 14 and supported on a spring 15 which is supported on a plate 17 which can be displaced by means of an adjusting screw 16. The baffle plate 13, together with the surrounding housing 18, has a fixing device consisting of a groove 19 and a snap pin 20 which can be pulled out of the groove 19 again by hand. The mold cavity is designated 21.

4 to 6:

A mixing head 41 combined with a mold 42. A housing 44 for a bore 45 is flanged to the molding box part 43. Inlet openings 46, 47 point into this bore 45. In the bore 45, an ejection piston 48 is guided, which in the mixed position releases the inlet openings 46, 47, so that between the ejection piston 48 and the outlet opening located at the end of the bore 45. 49 the mixing chamber 50 is formed. A mold parting plane 51 separates the two mold box parts 43, 52 from one another. A baffle plate 53 is guided on the molding box part 52 in the bore 54 of a flanged housing 58. It is mounted on a spring 55 which is supported on a plate 57 which can be adjusted by means of an adjusting screw 56. The mold 42 opens and closes perpendicular to the axis of the mixing chamber 50, so that the molded part to be manufactured is free of markings because the edge 60 of the baffle plate 53 facing the mold cavity 59 coincides with an edge of the molded part towards the end of the filling process. The mold parting plane 51 stands to the opening and closing direction of the mold 42 at an angle of 5 °. This results in a better seal. Arranged around the outlet opening 49 is a sealing surface 61 protruding from the mold parting plane 51, which corresponds to the end face shape of the baffle plate 53 and which, like the end face 63 of the baffle plate 53, corresponds to the opening and closing direction of the molding tool 42 at an angle of 1.5 ° is inclined to achieve a good seal. The back of the baffle plate 53 has an undercut 64 on the side facing the molded box part 52, which corresponds to a counter surface 65 of the housing 58 and ensures a good seal in the maximum open position.

Claims (6)

1. Process for the preparation of a reaction mixture which consists of at least two liquid reactants and forms a foam or homogeneous material, and for subsequent introduction of this mixture into a cavity (21), the reactants being fed into a mixing chamber (10) and mixed with one another, and the mixture then being passed through an outflow cross-section which can be adjusted automatically, even during the flowing-out process, and being filled into the cavity (21), characterised in that the mixing chamber (10) is closed before introduction of the reactants is started and is opened automatically, at a given pressure value, only the pressure which builds up in the chamber due to the introduction of the reactants.

2. Process according to Claim 1, characterised in that the size of the outflow cross-section is fixed towards the end of the flowing- out process.

3. Device for carrying out the process according to Patent Claim 1, which consists of mixing chamber (10) which is located in a housing (4) and has inlet openings (6, 7) for the reactants and an outlet opening (9) for the reaction mixture, this outlet opening (9) being located opposite a baffle (13) which is resiliently mounted and can be adjusted in the axial direction of the outlet opening (9), characterised in that, before the start of the mixing operation, the baffle (13) closes the outlet opening (9).

4. Device according to Claim 3, characterised in that the baffle (13) is mounted on a spring (15).

5. Device according to Claim 4, characterised in that the spring (15) rests on an adjusting screw (16) guided in the housing (18).

6. Device according to Claims 3 to 5, characterised in that the baffle (13) has a fixing device (19, 20).

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